The present invention relates generally to thermal decomposition furnaces for heating to high temperatures exhaust gases produced when general wastes such as resin and paper, industrial and medical wastes and crude oils, waste oils and petrochemical substances are burnt to thermally decompose harmful substances contained within the exhaust gases to render the substances harmless.
In waste processing facilities and factories, large amounts of various kinds of wastes/crude oils, waste oils and petrochemical substances are incinerated. Exhaust gases and smoke produced at that time contain harmful substances such as soot, dust, carbon monoxide, chlorine compounds such as hydrogen chloride, nitrogen compounds such as NOx, and dioxin, which affect adversely the environment and human bodies. Thus, the harmful substances in the exhaust gas and smoke are controlled. Especially, substances such as dioxin, which has a very high toxicity and adversely affects human bodies for a long time.
In general waste processing facilities and factories use incinerators of the type which feed air to wastes and petrochemical substances and burns them. These types of incinerators are called natural incinerators. Natural incinerators bum wastes at low temperatures, for example, at 300-500xc2x0 C., and cannot absolutely avoid production of dioxin.
In order to conform to control of dioxin discharge, the following measures are available:
(1) Wastes which produce no dioxin are segregated from those which produce dioxin, and only the wastes which produce no dioxin are burned;
(2) A device is used for removing or decomposing dioxin contained in exhaust gases discharged from an incinerator and is attached to the incinerator; or
(3) Incinerators are used which burn wastes at high temperatures (of about 800xc2x0 C.), where dioxin is difficult to produce.
However, there is the problem with the first method in that a great deal of time and a considerable cost are required for segregating the wastes. Complete segregation is substantially impossible and production of a small amount of dioxin cannot be avoided.
The second method is not satisfactory to cope with dioxin because there are no inexpensive devices for the complete removal or decomposition of dioxin.
The exhaust gases contain a plurality of harmful substances. In order to remove or decompose all of them, a plurality of devices is required to remove or decompose the harmful substances. Thus, there are the problems associated with the cost increases and complicated by the structure of the incinerator.
The third method provides for an incinerator which burns wastes at high temperatures and is expensive. This method has the problem in that the conventional incinerator must be replaced and a new incinerator which bums the wastes at the high temperatures installed.
Thus it is an object of the present invention to provide an inexpensive thermal decomposition apparatus which solves the abovementioned problems, and to provide an inexpensive thermal decomposition furnace for an exhaust gas attached to equipment/facilities such as an incinerator which discharges exhaust gases and smoke which contain harmful substances for thermally decomposing the harmful substances to make them harmless.
In order to achieve the above object, the present invention has the following composition. The present invention is a thermal decomposition furnace for an exhaust gas which thermally decomposes harmful substances contained in the exhaust gas to render them harmless, and comprises a heating chamber for heating the exhaust gas, an inlet port for introducing the exhaust gas into the heating chamber, at least one pair of electrodes provided within the heating chamber, a plurality of light emitting heaters whose main element is carbon, the plurality of light emitting heaters being provided between the at least one pair of electrodes so as to produce an electric discharge upon application of voltage across the at least one pair of electrodes, and an outlet port for discharging the decomposed exhaust gases out of the heating chamber.
In the above arrangement, electric discharges occur between the light emitting heaters. The electric discharge region is at high temperatures, for example, of about 3000xc2x0 C., so that a plurality of harmful substances such as carbon monoxide, chlorine compounds, nitrogen compounds and dioxin contained in the exhaust gas are simultaneously decomposed thermally at the high temperatures.
Exhaust gas thermal decomposition furnaces have a simple structure and can be made inexpensively. The furnace is the last furnace through which the exhaust gases pass from an existing or new incinerator. The harmful substances contained in the exhaust gases discharged from the incinerator are thermally decomposed and rendered harmless. Thus, even when the existing incinerator is of the type which produces a large amount of harmful substances, the furnace can be added without providing a new incinerator.
The light emitting heaters are preferably placed in an oxygenless environment. This prevents the oxidation of light emitting heaters and their deterioration thereby preventing a reduction in the discharging efficiency. Thus, the life of the light emitting heaters are extended. For example, when the plurality of light emitting heaters each take the form of a sphere, the discharging efficiency is very high. However, when the light emitting heater is oxidized, its deterioration and deformation reduce the discharging efficiency.
The oxygen concentration concerned is preferably low and there is no problem if it is not higher than the concentration of oxygen contained in air. When the oxygen concentration exceeds the concentration of oxygen contained in air, the plurality of light emitting heaters are liable to be oxidized and deteriorated.
Preferably, the plurality of light emitting heaters are placed in a vacuum. If it is placed in a clean environment such as the vacuum, the discharging efficiency is high and a corresponding high temperature is obtainable. Since a high temperature is obtained with little electric power, the operating cost of the decomposition furnace is low. In this case, as in the case in which the plurality of light emitting heaters are placed in the oxygenless state, they resist deterioration and have a long life.
A higher vacuum is preferred and a medium vacuum (of not less than 10xe2x88x922 Pa and less than 10 Pa) suffices, but a lower vacuum (of not less than 10 Pa and less than the atmospheric pressure) is usable.
Furthermore, a fluid path extends between the inlet and outlet ports allowing the exhaust gas to pass through. The path may be provided within the heating chamber with the plurality of the light emitting heaters being provided within at least a part of the fluid path.
In such an arrangement, the exhaust gas comes into direct contact with the plurality of light emitting heaters. Thus, the exhaust gas is heated at high temperatures, for example, of about 3000xc2x0 C. and almost all harmful substances are thermally decomposed.
A heat resistant pipe which places the inlet and outlet ports in fluid communication may be provided within the heating chamber with at least a part of the pipe encircled by the plurality of light emitting heaters.
In such an arrangement, the exhaust gas is heated within the heat resistant pipe and the exhaust gas does not come into contact with the light emitting heaters. Thus, corrosion and deterioration of the plurality of light emitting heaters by the exhaust gas does not occur.
Since the plurality of light emitting heaters are separated from the exhaust gas, they can be placed in an oxygenless or vacuum environment. Thus, the discharging efficiency is high and high temperature is easily obtained. High temperature is also obtained with a small electric power to thereby reduce the operating cost of the decomposition furnace. In addition, the plurality of light emitting heaters are not subject to deterioration and maintain a long life.
The heat resistant pipe may be made of a material which contains primarily carbon. In this case, since electric discharges occur also between the heat resistant pipe and the plurality of light emitting heaters which surround the heat resistant pipe, the heat resistant pipe is heated to a high temperature of about 3000xc2x0 C. As a result, the exhaust gas is thermally decomposed more efficiently. Carbon pipes are preferably used as the heat resistant pipes, which preferably have electric conductivity enough to cause efficient electric discharge.
The plurality of light emitting heaters may be made of charcoal or graphite. As an example of charcoal, Japanese Bincho charcoal is used. Carbon materials such as charcoal and graphite have many pores in its surface for adsorbing gases within the pores. There is the problem with the release of the adsorbed gases at high temperatures. Thus, carbon materials such as charcoal and graphite are processed to prevent gas adsorption by closing the pores.
Preferably, the plurality of light emitting heaters are impermeable. In this case, since there is a reduced absorptivity, there is a reduction in the adsorption of harmful substances contained in the exhaust gas. Since the light emitting heaters suffer less deterioration by the harmful substances contained in the exhaust gas or by oxidation, they can maintain a long life. The impermeability of a material implies that its absorptivity is low because the number of pores present in a surface of the material is small and that its specific surface area is small, contributing to low oxidation or corrosion by chemicals.
The light emitting heaters preferably take the form of a sphere. In order to have efficient electric discharges between the plurality of light emitting heaters, the light emitting heaters are preferably in point contact, one with another. If they are in a line or surface contact state, a high current flows thereby reducing the discharge efficiency. When the plurality of light emitting heaters each take the form of a sphere, they are necessarily placed in a point contact state, efficient electric discharge is performed thereby providing a high temperature, and reducing the operating cost of the decomposition furnace. The light emitting heaters take the form of a perfect sphere more preferably.
The decomposition furnace may comprise a filter made of active carbon or charcoal, allowing the thermally decomposed exhaust gas to pass therethrough. In this case, even if the decomposed gases contain hydrocarbons, heavy metals or undecomposed harmful substances, the filter will adsorb them to prevent them from being discharged from the furnace.